As technologies for controlling a directional beam using an array antenna, which is an arrangement of a plurality of antenna elements, analog control which uses a phased array, and digital control in which each antenna element is weighted through digital signal processing are generally known. Because the analog control uses one chain of a radio circuit and a digital-to-analogue (DA)/analogue-to-digital (AD) converter for a plurality of antenna elements, power consumption by the analog control is relatively low. With the analog control, however, because a directional beam (hereinafter, simply referred to as a “beam”) is formed in one direction, it is difficult to steer beams to a plurality of users simultaneously. By contrast, beams can be easily steered to a plurality of users simultaneously with the digital control, while the digital control uses a plurality of chains of a radio circuit and a DA/AD converter, in the number equal to the number of antenna elements, which results in increasing of the power consumption by the digital control.
To address this issue, hybrid control, which uses a combination of the analog and the digital beam control, has been recently developed. With the hybrid control, beams can be steered to a plurality of users simultaneously while consuming less power than that in the digital control. In other words, although the hybrid control uses a plurality of chains of a radio circuit and a DA/AD converter, each of the chains including the radio circuit and the DA/AD converter is connected to a plurality of antenna elements. Therefore, a smaller number of chains of the radio circuit and the DA/AD converter, being smaller than the number of antenna elements, is used, and less power is consumed compared with the digital control. Furthermore, because a plurality of data streams in the number equal to the number of chains of the radio circuit and the DA/AD converters can be processed simultaneously, beams can be steered to a plurality of users in the number equal to the number of chains of the radio circuit and the DA/AD converters simultaneously.
[Patent Document 1] Japanese National Publication of International Patent Application No. 2014-527749.
[Non-Patent Document 1] Xiaojing Huang et al., “A Hybrid Adaptive Antenna Array”, IEEE TRANSACTIONS ON COMMUNICATIONS, Vol. 9, No. 5, pp. 1770-1779, May 2010.
However, the hybrid beam control is, disadvantageously, not quite capable of steering beams to a plurality of users at the same power strength. In other words, because the hybrid control uses the combination of the analog control and the digital control, each of the antenna elements is weighted based on the phase difference between the antenna elements, based on the same scheme as that used in the analog control. The digital signal processing then adjusts the beams having been formed by weighting so that the beams can be steered to a plurality of users simultaneously. At this time, because the beam adjustment through the digital signal processing has a certain limitation, the beams resultant of the digital signal processing deteriorate, compared with the beams formed by weighting the antenna elements.
In this manner, in the hybrid beam control, because a beam corresponding to the analog control is more dominant, the power of a beam corresponding to the digital control may be weaker than that of the beam corresponding to the analog control. The hybrid beam control, therefore, is sometimes incapable of steering a beam to a user at sufficient power strength, depending on where the user is located, and the communication quality for the user may deteriorate.